Heavy duty wood chippers that are capable of producing large amounts of chips in a relatively short period of time typically employ a circular disc that is mounted for rotation upon a horizontally disposed drive shaft. A plurality of rectangular knives are radially extended upon the front face of the disc. Each knife is, in turn, aligned adjacent to a chip slots that passes through the disc between its front face and back face. Logs or similar work pieces are brought into contact with the front face of the rotating disc within a stationary work station where the moving knives act in association with a stationary bed knife to remove chips from the work pieces. The chips are conducted through the chip slots and exhausted from the machine.
The knives that are typically used in these heavy duty machines are subjected to high impact loads which, if not effectively resisted can adversely affect the life of the knives and quality of the product produced in the machine depending upon various loading factors. In an effort to combat these high loads, it has been customary in the industry to use knives that are relatively massive in not only size but also in weight. As a result, the apparatus for securing the knives within a chipper is generally rather complex and expensive to build and maintain. Removal of the knives from the machine for refurbishing or replacing is usually a time consuming procedure. In addition, handling the massive knives is a multi-worker task and can pose a danger to the workers and the knives if the knives are mishandled or dropped. Systems employing much smaller knives meanwhile typically compromise the strength of the knives and the holding system resulting in a weaker overall clamping and holding system.
Good knife support systems must also accommodate normal variations in thickness of knives due to variability in knife manufacture tolerances and also to accommodate for contamination such as wood dust, tree sap and debris that might work itself between mated parts while still safely and adequately holding the knives in place.
The apparatus used to secure a chipper knife to a disc most generally involves some type of clamping mechanism for releasably capturing the knife or knives between the clamp and a substrate on the disc. The clamping force is typically provided by bolts that pass through the clamp and are threaded into the substrate so that the axes of the bolt shanks are generally normal to the front face of the disc. When used with hardware that permits keyed or doweled contact and transmission of forces to the disc, this type of clamping arrangement is well suited to resist input loads acting parallel to or perpendicular with the axis of the bolts. The clamping arrangement, however, is not as well suited to resist loads other than those running along these two primary axes or to couples arising from loads not being applied concentrically with the disc key supports or the holding bolts. These off-axis loads and couples are generally concentrated along one edge section of the bolt heads, a condition referred to as “point loading” of the bolt head, which results in a reduction in the holding effectiveness of the clamp and clamping bolts. When point loading occurs, the bolts or knife clamps can fatigue and fail giving rise to an unsafe situation of inadequately clamped knives and knives rapidly become dull or damaged and the quality of the chips produced being greatly reduced. Gaps and clearances that are often present between clamping system components can permit parts to deflect from their stress-free manufactured states, further exacerbating the “point loading” problem referred to above and observed in practice.